The present invention relates to defect detection in and error recovery from magnetic media. More particularly, the present invention relates to detection finding and error recovery using variable level qualification from a raw data signal generated by movement of a magnetic transducer relative to the surface of a magnetic disk.
A magnetic disk, such as used in a computer disk drive, is a flat circular platter with a magnetic surface on which data can be stored by selective polarization of portions of the magnetic surface. The presence or absence of polarity transitions between the polarized portions represents particular binary values. Typically, the magnetically polarized portions are arranged in a plurality of radially concentric tracks on the surface to aid in location and readback of the data.
A magnetic transducer moving relative to the magnetic disk along a given track generates an electrical signal (the "read signal"), which is related to the states of polarization encountered along the track. Pulses in the read signal, i.e. brief excursions in the signal value from its normal or initial level, correspond to the magnetically polarized portions of the magnetic disk. Ideally, the read signal would not be influenced by any other factors. Unfortunately, other factors must be taken into account. In the read signal, data is indicated by pulse polarity transitions. However, not all pulses appearing in the read signal are true (and some pulses may be missing) thus sorting the data from the noise has become a major concern in signal processing for magnetic disk drives.
Read signal strength varies with the strength of the magnetic flux density encountered by the magnetic transducer as it moves across the magnetic disk. The strength of the encountered magnetic flux density in turn depends upon the spacing of the transducer from the surface of the magnetic disk, the orientation of the transducer relative to the tracks, the accuracy of positioning the transducer relative to the data tracks, the data coding scheme employed and many other factors.
The above factors can also affect the strength of the polarization of the magnetic surface where the same transducer is used to write data to the disk. For example, increased spacing between transducer and surface reduces the strength of polarization from writing to the disk. An increase in spacing between transducer and surface will show up in the read signal as a weakening in the signal. Thus, the effects can be additive.
Automatic gain control has long been used to solve the problems relating to variation in basic signal strength. However, the problems of data reproduction become more severe with increasing data densities, and involve factors not compensated effectively for by automatic gain control, particularly defects in the magnetic disk, crosstalk and intersymbol interference. These problems can result in the appearance of false pulses in the readback signal.
Increases in the areal density of magnetically polarized portions on magnetic disks lead directly to increases in data storage capacity for a disk drive of a given size. However, increased storage densities lead to an increase in susceptibility of the read signal to crosstalk and intersymbol interference. Coding schemes directed toward increased data densities, moreover, can be expected to produce significantly more occasions where there are multiple peaks occurring above a static lowered qualification threshold or peaks located in the long baseline between widely separated pulses. Codes that employ wide "windows" in order to increase disk capacity have larger band widths and longer baselines leading to intersymbol interference and crosstalk susceptibility. The intersymbol interference and crosstalk problems are further exacerbated if the magnetic read head drifts slightly off track. Transducer orientation and spacing from the magnetic media surface also effect crosstalk and intersymbol interference problems. These factors decrease the signal-to-noise ratio of the read signal and make determination of which pulses are true ever more difficult. False pulses, e.g. pulses related to transducer pickup of crosstalk, and missed true pulses or defects in the magnetic disk cause data decoding problems, and can result in inconsistent operation of a phase locked loop used to recover the "clock" or timing of the data.
A technique for eliminating many false pulses from a raw data signal is to subject the raw data signal to pulse threshold qualification. Threshold qualification requires that a pulse in the raw data signal exhibit a predetermined minimum signal level (qualification level) as one step in qualification of the pulse as one having a high likelihood of being a true pulse. In the prior art, a single qualification level has been set for an entire magnetic disk, or even an entire stack of magnetic disks in multiplatter applications. The qualification level has been selected so that there are an equal number of "dropouts" and "extra" pulses due to the noise present in a raw data signal.